It is the function of the combustor section of a gas turbine engine to completely react the engine fuel and compressed air delivered from the upstream combustor section prior to discharging the heated combustion products into the downstream turbine section. Typical combustors contain the engine working fluid in an annular region defined by inner and outer engine case walls, while the fuel and air are mixed and reacted within one or more combustion chambers located within the annular region.
A typical combustion chamber is defined by an air cooled liner which includes a plurality of openings for admitting pressurized air delivered by the upstream engine compressor section, and at least one fuel nozzle for delivering a flow of combustion fuel. The gas dynamics within the combustion chamber is extremely complex, as the designer attempts to maximize mixing, flame stability, turndown ratio combustion efficiency, and pressure loss within a limited space. Mixing and flame stability are, in larger engines, achieved by directing a substantial fraction of the compressed air into the combustion chamber through louvers or openings located about the periphery of the larger opening through which the fuel nozzle penetrates the combustor liner. This nozzle air flow is usually swirled or otherwise vectored so as to create an immediate zone of recirculation in the vicinity of the discharged fuel stream within the combustor. The recirculating air and combustion products stabilize the reacting fuel air mixture within the combustor, preventing flameout or other instabilities. The rapidly swirling or recirculating air mixture also enhances dispersion and reaction of the fuel within the chamber, assisting in causing the fuel and air to complete the combustion reaction prior to exiting the chamber.
The use of an individual air swirler for each fuel nozzle is common, if not necessary, in combustor arrangements wherein a plurality of individual combustion chambers are located within the annular combustor zone, with each chamber having a single corresponding fuel nozzle. The use of individual swirlers is also quite common in larger gas turbines wherein a single annular combustor arrangement is used, but has proved less desirable for small gas turbine engines wherein space considerations make it difficult to incorporate an individual air swirler for each nozzle. Another factor to be considered in the design of a combustor for a gas turbine engine is the ability of such combustor to accommodate varying flows of fuel and air while maintaining stable performance.